1. Field of the Invention
The present invention relates to an imaging optical system which uses a solid-state image sensor, and particularly, relates to a compact high-quality imaging optical system having a fast aperture ratio of 1:2.8 through 3.5, and a half angle-of-view of 30° or more. The above-mentioned imaging optical system is suitable for a small and light-weight digital still camera and a video camera, and in particular, suitable for a mobile phone and a PDA (portable data handsets) in which the imaging optical system has to be accommodated into a very thin body.
2. Description of the Prior Art
In recent years, a conventional silver-halide film camera has been replaced with a digital still camera and a video camera, both of which are hereinafter referred to as a digital camera, utilizing a small solid state imaging sensor such as CCDs and CMOSs, etc. Such a digital camera has rapidly become popular.
Along with the development of a miniaturized solid state imaging sensor with higher density of pixels, a digital camera has had higher optical performance, such as higher resolution and a higher zoom ratio.
On the other hand, a mobile phone and a PDA, both of which are hereinafter referred to as a portable device, have introduced a portability-oriented design. Furthermore, the portable device provided with a photographic lens unit in order to function as a digital camera has rapidly become popular.
In a small digital camera, an attempt has been made to attain a thinner body thereof by mechanically retracting the photographic lens unit into the camera body when the digital camera is not used.
However, since the industrial standards for the portable devices with respect to falling and shock, etc., are strict, it is difficult to employ a mechanism to protrude a portion of the camera body (e.g., the photographic lens unit) from the main body thereof, and to employ a drive mechanism to mechanically drive the photographic lens unit, etc., when the digital camera is being used.
Accordingly, most of the portable devices have merely employed a smaller solid-state image sensor and a fixed-focal length optical system including only one or two lens elements.
Therefore it has been known that such a fixed-focal length optical system can at most obtain a low resolution image just for a user's temporal memorandum. In recent years, on the other hand, an optical system incorporated in the portable devices has been required to have a higher resolution equivalent to a digital camera.
In a solid state imaging sensor used in the above-mentioned digital camera, a micro-lens element and a color filter are provided in the close vicinity of the light-receiving surface (imaging surface) of pixels in order to increase aperture efficiency of the light-receiving portion of the solid state imaging sensor. However, the micro-lens element and the color filter are generally positioned slightly away from the imaging surface. Therefore if light rays emitted from the final lens element are obliquely incident on the imaging surface, the oblique light rays are interrupted by the filter, so that shading occurs. The shading causes a decrease of peripheral illumination and unevenness of color distribution due to misalignment between the color filter and the pixels.
In order to avoid the above drawbacks, the imaging optical system is required to have telecentricity, i.e., the light rays are made incident on the imaging plane in a direction substantially perpendicular thereto. In other words, the imaging optical system in which the exit pupil is positioned far away from the image plane has been required.
The solid-state image sensor requires a space for positioning at least the following optical elements:
(i) a protective glass plate for preventing scratches on the imaging surface, and dust thereon;
(ii) an optical low-pass filter for preventing moiré caused by the periodic structure of the solid-state image sensor; and
(iii) an infrared-cut filter for lowering sensitivity of the infrared wavelength range so that sensitivity corresponding to the visible wavelength range can substantially be obtained.
Still further, a longer back focal distance (the distance from the final lens surface to the imaging surface) is required to prevent shading caused by dust sticking to a lens surface.
In order to attain higher resolution, it has been common to increase the number of pixels by miniaturizing each pixel while the dimensions of the imaging surface remains the same. Furthermore, in recent years, the pixel pitch has been gradually approaching the wavelength of visible light. One pixel pitch up to 2.5 μm has already been achieved, and further miniaturization of the pixel pitch has been getting close to its limit.
Accordingly, in order to increase the number of pixels, it has becomes essential to increase the dimensions of the imaging surface. Increasing the dimensions of the imaging surface is equivalent to making the focal length of the imaging optical system longer. However, it is known that aberrations become larger in proportion to the focal length. Therefore the imaging optical system has to cope with a lot of optical requirements.
Recently, optimization on the positions of the color filters and micro-lens elements which are used with CCDs or CMOSs and the like has become possible in accordance with a type of the imaging optical system, so that requirements for telecentricity of the imaging optical system are not so strict as they used to be.
Furthermore, the low-pass filter provided between the imaging optical system and the solid-state image sensor can be omitted due to further development of image processing technology through which a higher processing speed can be attained. Namely, the role of the low-pass filter can be replaced with the image processing technology itself.
Accordingly, along with technological advancements in the solid-state image sensor and other related technologies, a compact imaging optical system, having an appropriate telecentricity, the back focal distance to the minimum necessary, and higher resolution, has been more and more required.
As a compact imaging optical system having superior portability, a lens system of single-lens-element arrangement or two-lens-element arrangement has been known in the art. Furthermore, an aperture stop is provided on the object side of the above lens system.
However, it has been difficult for such imaging optical systems having a single or two lens-element arrangement to be considered as an imaging optical system which attains a higher picture quality and a higher resolution.
In order to solve the above drawbacks, an imaging optical system, including an aperture stop provided on the most object side thereof, and a three-lens-element arrangement (a positive refractive power (hereinafter, a positive lens element), a lens element having a negative refractive power (hereinafter, a negative lens element), and the other positive lens element) has been proposed in Japanese Unexamined Patent Publication (JUPP) No. Hei-5-188284 and JUPP No. 2001-75006. However, in such an imaging optical system in which the refractive power is distributed over the positive lens elements at the most object-side and the most image-side of the imaging optical system, the back focal distance can be secured relatively longer; however, in the case where the optical low-pass filter and the infrared-cut filter are omitted, the distance from the aperture stop to the imaging surface becomes undesirably too long.
Furthermore, since the refractive power of each lens element is designed to be stronger, when a resin material is used for forming the lens elements for the purpose of cost reduction, it should be understood that the lens elements made of a resin material are vulnerable to the changes in environmental conditions such as temperature and humidity.
In order to solve the above drawbacks experienced in JUPP No. Hei-5-188284 and JUPP No. 2001-75006, JUPP No. 2002-228922 and JUPP No. 2002-365529 have proposed an imaging optical system with the following features:
(i) a strong positive refractive power is given to an object-side lens element(s);
(ii) a relatively weaker refractive power is given to an image-side lens element(s); and
(iii) the image-side lens element(s) is arranged to only function as an aberration-correcting lens element(s).
Due to the above arrangement, the distance from the aperture stop to the imaging surface can be made shorter; however, the imaging optical system is constituted by four lens elements. If an attempt is made to make the imaging optical system shorter, each lens element has to be made thinner, so that machining the lens element becomes difficult, and manufacturing costs thereof increase.